1. Field of the Invention
The present invention relates to a switching power supply device, and particularly to an uninterruptible switching power supply device, which is designed to be energized by an AC power source and provided with a DC power source to compensate DC output during brownout or failure of the AC power source.
2. Background of the Prior Art
Conventionally, a switching power supply with favorable voltage transformation efficiency has been widely used in various products such as computers and communication devices. With the advert of computers and their sensitivity to input line voltage (AC power) variations and transients appearing thereon, it has become necessary to provide a regulated DC power source for operation of these computers that is not subject to temporary AC input power failure or intermittent brown-out conditions. However, because of lacking in such a function as uninterruptibly powering a load, those precise electronic equipments are usually still provided with an additional UPS in addition to a switching power supply, which increases the cost.
Chinese patent No. 94221822.1, entitled “An Online Uninterruptible Switching Power Supply Device with Smart Energy Compensation”, discloses an uninterruptible switching power supply device. The device comprises a main transformer, a battery, a high voltage switching circuit, a low voltage switching circuit and a pulse width modulator (PWM). The high voltage switching circuit and the low voltage switching circuit both of which are arranged at the side of the primary winding of the main transformer and independent from each other are controlled by the PWM to operate synchronously. The operation principle of the known switching power supply device is inducted energy transformation. However, the efficiency of the inducted energy transformation will be decreased when the voltage of AC input power for energizing the device becomes low. Moreover, the leakage of the magnetic flux existing in the known device results in serious disturbance.